What is stainless steel surface passivation
Passivation of stainless steel surfaces refers to the use of acid solutions to remove free iron from stainless steel surfaces to prevent rust. When the surface is treated, the other components of the alloy (mainly chromium, often also nickel) are left as a surface layer on the underlying steel. When exposed to air, these elements react with oxygen to form an oxide layer that protects the rest of the steel from corrosion. This corrosion-resistant surface can be damaged by mechanical means or thermal or chemical damage. When this happens, the iron is exposed and the stainless steel will rust again.
Process principle of stainless steel surface passivation
Stainless steel passivation has details such as the exact mix of chemicals used that can vary depending on the steel alloy used. What helps protect one alloy can damage another, so the process often needs to be customized . However, the basic process tends to have the same process steps:
Load the parts to be passivated into another container.
Immerse the container and its contents in a chemical bath (usually a citric or nitric acid bath).
Allow the part to sit in the chemical bath for a period of time and at the temperature specified for the alloy.
Remove the part from the chemical bath and dry it to remove excess chemical.
As stainless steel parts sit in an acid bath, the acid will begin to strip free iron and other contaminants from the surface of the part. Depending on the alloy being cleaned and the specific chemical bath used, this can strengthen the protective oxide layer of the stainless steel component, making it in some cases more resistant to chemical attack.
While the passivation process can significantly alter the rust resistance of a part, it does not significantly change the appearance of the finished part. This is one of the key differences between passivation and electropolishing. Components that have been electropolished can be easily identified by their smooth, shiny surface.
Advantages of stainless steel surface passivation
1. Improve corrosion resistance.
2. Uniform and smooth appearance and surface.
3. Deburring (polishing surface).
4. High cleanliness.
5. Improve and prolong product life.
Disadvantages of stainless steel surface passivation
Before passivating stainless steel parts, some things should be considered, such as:
Soldered parts may not be suitable for passivation because the process is not as effective as some other methods at stripping contaminants from the welded area.
Requires custom chemical bath. The temperature and type of acid used in the passivation process chemical bath must be adjusted to the specific steel alloy being passivated. This adds cost and complexity to the method compared to electropolishing.
Some alloys cannot be passivated. Some stainless steel alloys with low chromium and nickel contents can actually be damaged by acid baths. Therefore, they cannot be passivated.
Unlike electropolishing, which strips the surface layers of a part to leave a smooth, shiny surface, passivation does not alter the appearance of the part. So if the goal is to create a smooth, non-stick surface, the passivation process won't be ideal. The main advantages of stainless steel passivation are that it enhances the rust resistance of stainless steel components, and it is less expensive than electropolishing.
